The use of an aforementioned CARS method and/or CARS optical arrangement has become established in particular in the region of investigating internal vibronic structures of molecules, wherein for example a non-linear optical signal in the form of a CARS signal (Coherent Anti-Stokes Raman Spectroscopy Signal) can be produced on a material. By way of example the article by F Ganikhanov et al, in: Optics Letters, Vol 31 (12), pages 1872-1874 (2006) describes a high-sensitivity CARS structure which operates with two lasers and a resonator-internal frequency-doubled OPO. That permits marker-free identification of a molecule. Thus for example fluorescence spectroscopy markered species as possible alternatives but in particular in the biological or foodstuffs sector are found to be not application-related by virtue of the frequently excessively low marker action and possibly toxic action of the markers. The method and the apparatus set forth in the opening part of this specification thus form a promising and viable option if it is possible to reduce the complexity of existing systems and at the same time enhance the sensitivity thereof. The resonant elevation inherent in the CARS process makes it more sensitive than Raman spectroscopy and in comparison with infrared spectroscopy the use of shorter wavelengths in the present case permits a higher level of spatial resolution in the region of microscopy. For that reason amongst others the use of a method as set forth in the opening part of this specification and the above-mentioned apparatus in the context of CARS spectroscopy and microscopy has acquired a not inconsiderable significance. The implementation thereof on a larger, application-related or industrial scale has hitherto been limited by virtue of the still comparatively high degree of complexity of the systems required for same and the comparatively low yield of the non-linear optical signal which is generated in the material by means of an available interaction which is non-linear to a high degree. Those problems arise in particular in relation to materials in which the substance to be investigated is present in a comparatively low concentration.
Thus the publication by E O Potma et al in Opt Lett 31, No 2, pages 241 through 243, 14 Jan. 1006 ‘Heterodyne coherent anti-Stokes Raman scattering (CARS) imaging’ and the publication by E R Andresen in Opt Expr Vol 14 (16), pages 7246-7251 (2006) ‘Picosecond anti-Stokes generation in a photonic-crystal fiber for interferometric CARS microscopy’ basically propose developing a multi-beam arrangement as referred to in the opening part of this specification for a method and an optical structure for generating the non-linear optical signal by the use of an optical parametric oscillator and a heterodyne detection system. For that purpose E O Potma et al propose, for a CARS arrangement, using a pulse of a signal beam, which is frequency-doubled in the resonator of the optical parametric oscillator (OPO), as a high-frequency excitation field, and using a laser pulse generated by a pump laser for the OPO as a low-frequency excitation field, in the CARS terminology as the Stokes beam. To implement a heterodyne detection system an additional CARS structure is made available in a defined manner in the CARS structure, in the context of a Mach-Zehnder interferometer, wherein one of the arms of the interferometer has a cell of deuterated dimethylsulfoxide (d-DMSO) for generating a strong non-resonant signal at a—in the CARS terminology—anti-Stokes frequency, which serves as a so-called ‘local oscillator’ in accordance with the heterodyne detection system. Such a structure is comparatively complex and complicated to handle although the proposed heterodyne detection system is basically suitable for increasing a level of sensitivity of the aforementioned method and the aforementioned apparatus.
In the publication by E R Andresen et al once again a signal output of an OPO synchronously pumped to a laser frequency serves as a high-frequency pump pulse for the excitation field for generating a CARS signal while a laser pulse at the laser frequency serves as a low-frequency Stokes pulse for forming the excitation field. Pump and Stokes pulses are fed to a photonic-crystal glass fiber (PCF) to represent a local oscillator field in the anti-Stokes pulse in accordance with the heterodyne detection system for CARS. The dispersion and transit time properties linked thereto concerning the anti-Stokes pulse give rise to additional problems which make the method described therein and the structure described therein still worthy of improvement.
The publication ‘CARS imaging with a new 532 nm synchronously pumped picosecond OPO’ by Bfttner et al, which appeared in Proc of SPIE Conf Multiphoton Microscopy in the Biomedical Sciences VII (21-23 Jan. 2007), edited by A Periasamy et al, SPIE, 2007 Vol 6442, pages 64420C-1 through 64420C-8), basically describes an advantageous optical CARS structure with an OPO, which is pumped with a frequency-doubled, mode-coupled Nd: VAN laser. There a signal beam of the OPO is used together with a part of the fundamental of the pump laser as a pump and Stokes beam on the implementation of a CARS excitation field.